Ecological Studies

Question 1

what is an ecological study

Answer 1

observational study where data is analysed at population or group level rather than individual
area-level studies to measure prevalence and incidence of disease - particularly when it is rare

Question 2

what are the advantages of ecological studies

Answer 2

inexpensive
easy to do as use routinely collected data
population context of individual characteristics is a stronger determinant of disease at population level than individual level risk factors

Question 3

what are the disadvantages of ecological studies

Answer 3

prone to bias and confounding
care must be taken when extrapolating to individuals within the measurement areas
or to higher population level
other designs are generally considered more reliable in respect to inference and causation

Question 4

why would someone do an ecological study

Answer 4

to monitor population health
informing and develop public health strategies
for large scale comparisons e.g between countries
to study the relationship between population-level exposure to risk factors and disease
or to study contextual effect of risk factors on population
if individual measurements are not available
if the disease is rare

Question 5

whats an example of when individual measurements may not be available and one has to do an ecological study?

Answer 5

confidentiality precludes the release of data on individuals
so its anonymised by aggregation of data to small area level

Question 6

what are the types of measurement

Answer 6

health outcomes (incidence/prevalence/rate)
aggregate measures (mean/%/area-level deprivation indices)
environmental measures
global measures (e.g number of GPs, or population density)

Question 7

what is a geographical ecological study

Answer 7

compares one geography to another and assesses health of each
exposure may be measured and included in analysis
also possible confounders
e.g demographic, socioeconomic

Question 8

what is a longitudinal ecological study

Answer 8

population monitored to assess changes in disease over time
confounding factors may be included in analysis

Question 9

what is a migration ecological study

Answer 9

migrant population data collected and analysed
unit of interest is population type
not time or place

Question 10

what is an example of ecological fallacy

Answer 10

falsely assumes that every individuals IQ is high, , since the class average is high
e.g if students in class got 50, 90, 130 and 150 would average 105

Question 11

what is ecological fallacy

Answer 11

type of confounding specific to ecological studies - occurs when relationships found for groups are assumed also to be true for individual
- may be unknown confounding factors that display same pattern of spatial heterogeneity

Question 12

why does ecological fallacy happen

Answer 12

the data is aggregated so does not provide exact information on those particular individual

Question 13

how do we overcome ecological confounding?

Answer 13

regression modelling
poisson regression
negative binomial regression
multilevel modelling

Question 14

what are area-level exposures

Answer 14

environmental exposures determined by area
e.g in air or drinking water
individual exposure to these things associated with area of residence work, study - reflects demographic and lifestyle factors - age, sex , SES

Question 15

what is the ideal analysis and problem with it

Answer 15

based on individual-level data but not feasible or very expensive to obtain

Question 16

what can be done instead of individual-level data

Answer 16

need to model exposures
but proxy measures based on models may not capture exposure of individual

Question 17

what can modelling exposures lead to

Answer 17

exposure misclassification

Question 18

what does exposure misclassification often lead to

Answer 18

it is non-differential to disease status so leads to
regression dilution bias - means the bias of effect size estimates towards the null

Question 19

what is the excessive relative risk of environmental exposures expected to be

Answer 19

small

Question 20

what could non-differential exposure mis-classification leading to a bias toward the null lead to

Answer 20

false-negative results
false reassurance about the health effect of exposure

Question 21

what is the implication to public health of non-differential exposure misclassification

Answer 21

even small excess relative risks applied to large numbers of people could result in large excess numbers of disease
consequential for rare diseases - e.g childhood leukemia

Question 22

outline the bone cancer example of ecological study

Answer 22

aim to test prediction of spatial variations that may arise as result of environmental mechanisms and SES deprivation
based on the density of population of area and SES
patients diagnosed 1980-2005, data from registry

Question 23

what were the 4 census measures in the bone cancer study?

Answer 23

unemployment
households with no car
non-home ownership
household overcrowding

Question 24

what were the findings of the bone cancer study

Answer 24

females - lower incidence of osteosarcoma associated with higher deprivation
lower incidence of Ewing sarcoma associated with living in more densely populated area and higher non-car ownership

Question 25

what can be concluded for females and osteosarcoma in the bone cancer study?

Answer 25

geographical heterogeneity of incidence is modulated by differences in environmental exposure occurring in less and more socio-economically deprived areas of residence

Question 26

what can be concluded for Ewing sarcoma in the bone cancer study?

Answer 26

geographical heterogenity of incidence is modulated by differences in environmental exposures occurring in less and more densely populated areas
also modulated by differences between environmental exposure occurring in less and more socioeconomically deprived areas

Question 27

outline the fluoride study

Answer 27

it is in drinking water - possibility of bone cancer?
examine if increased risk of bone cancer is linked to residence in areas with higher fluoride in water
hypothesis - modulated for osteosarcoma but NOT Ewing sarcoma
statsitical analysis - negative binomial regression - with adjustments for previously identified deprivation

Question 28

what were the findings of the fluoride study

Answer 28

no evidence for the association between fluoride in water and osteosarcoma
no evidence for Ewing sarcoma either

Question 29

why may there have been no association found in the fluoride study?

Answer 29

because of attenuation due to exposure measurement error - arising through imprecision of allocating fluoride levels
the study was ecological and so may not reflect individual exposures

Question 30

what is the type 1 diabetes study

Answer 30

incidence relationship between childhood type 1 and levels of nitrate in drinking water
yorkshire
statsitical model was poisson instead of negative binomial

Question 31

what did the type 1 diabetes study find

Answer 31

diabetes incidence was positively associated with raised mean nitrate levels
significant negative trend found between standardised incidence ratio and proportion of non-caucasians in population, and population density

Question 32

what did the type 1 diabetes study conclude

Answer 32

findings were not explained by ethnic composition of pop, pop density or SES
nitrate in drinking water may be a precursor of chemicals toxic to the pancreas